The removal of finely particulate surface contamination has been the subject of numerous investigations, especially in the semiconductor industry. Large particles, i.e. in excess of one micron, are easily removed by blowing with a dry nitrogen stream. However, submicron particles are highly resistant to removal by gaseous streams because such particles are more strongly bound to the substrate surface. This is due primarily to electrostatic forces and bonding of the particles by surface layers containing absorbed water and/or organic compounds. In addition, there is a boundry layer of nearly stagnant gas on the surface which is comparatively thick in relation to submicron particles. This layer shields submicron particles from forces which moving gas streams would otherwise exert on them at greater distances from the surface.
It is generally believed that the high degree of adhesion of submicron particles to a substrate is due to the relatively large surface area of the particles which provides greater contact with the substrate. Since such particles do not extend far from the surface area and therefore have less surface area exposed to the stream of a gas or liquid, they are not easily removed by aerodynamic drag effects as evidenced by studies of the movement of sand and other small particles. Bagnold, R. The Physics of Sand and Desert Dunes, Chapman and Hall, London (1966) pp 25-37; and Corn, M. "The Adhesion of Solid Particles to Solid Surfaces", J. Air. Poll. Cart. Assoc. Vol 11, No. 11 (1961) pp 523-528.
The semiconductor industry has employed high pressure liquids alone or in combination with fine bristled brushes to remove finely particulate contaminants from semiconductor wafers. While such processes have achieved some success in removing contaminants, they are disadvantageous because the brushes scratch the substrate surface and the high pressure liquids tend to erode the delicate surfaces and can even generate an undesirable electric discharge as noted by Gallo, C. F. and Lama, W. C., "Classical Electrostatic Description of the Work Function and Ionization Energy of Insulators", IEEE TRANS. IND. APPL. Vol 1A-12, No. 2 pp 7-11 (January/February 1976). Another disadvantage of the brush and high pressure liquid systems is that the liquids can not readily be collected after use.
In accordance with the present invention, a mixture of substantially pure solid and gaseous carbon dioxide has been found effective for removal of submicron particles from substrate surfaces without the disadvantages associated with the above-described brush and high pressure liquid systems.
More specifically, pure carbon dioxide (99.99+%) is available and can be expanded from the liquid state to produce dry ice snow which can be effectively blown across a surface to remove submicron particles without scratching the substrate surface. In addition, the carbon dioxide snow vaporizes when exposed to ambient temperatures leaving no residue and thereby eliminating the problem of fluid collection.
Ice and dry ice have been described as abrasive cleaners. For Example, E. J. Courts, in U.S. Pat. No. 2,699,403, discloses apparatus for producing ice flakes from water for cleaning the exterior surfaces of automobiles. U. C. Walt et al, in U.S. Pat. No. 3,074,822, disclose apparatus for generating a fluidized frozen dioxane and dry ice mixture for cleaning surfaces such as gas turbine blades. Walt et al state that dioxane is added to the dry ice because the latter does not evidence good abrasive and solvent action.
More recently, apparatus for making carbon dioxide snow and for directing a solid/gas mixture of carbon dioxide to a substrate has been disclosed. Hoenig, Stuart A., "Cleaning Surfaces with Dry Ice" (Compressed Air Magazine, August, 1986, pp 22-25). By device, liquid carbon dioxide is depressurized through a long, cylindrical tube of uniform diameter to produce a solid/gas carbon dioxide mixture which is then directed to the substrate surface. A concentrically positioned tube is used to add a flow of dry nitrogen gas to thereby prevent the build-up of condensation.
Despite being able to remove some submicron particles, the aforementioned device suffers from several disadvantages. For example, the cleaning effect is limited primarily due to the low gas velocity and the flaky and fluffy nature of the solid carbon dioxide. In addition, the geometry of the long cylindrical tube makes it difficult to control the carbon dioxide feed rate and the rate at which the snow stream contacts the substrate surface.
In accordance with this invention, there is provided a new aparatus for removing submicron particles from a substrate which overcomes the aforementioned disadvantages. The apparatus of this invention produces a solid/gas mixture of carbon dioxide at a controlled flow rate which effectively removes submicron particles from a substrate surface.